Abstract

An epigenetic replacement model is proposed for Witwatersrand gold mineralization. In this model, unconformities that developed during deposition of the Upper Witwatersrand sediments were sites of selective accumulation of iron pisolites-ferricretes and carbon seams. These Fe-C-bearing intervals are part of 1- to 2-m-thick reef packages that were preferred channelways for Au-S-bearing metamorphic fluids because of their lithologic heterogeneity and preferential accommodation of strain during deformation. It is suggested that the metamorphic fluids were associated with a high-strain, cleavage-forming event recorded in the Witwatersrand, Venters-dorp, and Transvaal successions.

Selective replacement of Fe-C-bearing phases by Au-S-bearing metamorphic fluids accounts for the secondary nature of the Witwatersrand mineralogy, apparent structural control on gold mineralization, the secondary texture of gold, and the presence of replacement textures involving pyrite and uraninite. Most importantly, the model also accounts for the close relationship of gold distribution to sedimentary features. If the present distribution of Fe and C approximates original sedimentary patterns, any epigenetic gold precipitated by interaction with Fe and/or C must also reflect sedimentary features.

This replacement model obviates the requirements imposed by the placer model for Witwatersrand gold for a more reducing Precambrian atmosphere, for a particularly enriched source terrane, or for an unusually efficient sorting mechanism to concentrate gold in a diverse range of rock types.

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